After the persian war, what did sparta, athens, and other greek communities collect to ward off future external attacks?
slaves
corn and other grains
money
resources for building walls

The correct answer to this question would be alternative D)" Resources for building walls."

After the Persian War, Sparta, Athens, and other Greek communities collected resources for building walls to ward off future external attacks.

Even though they needed money, which is alternative C, money itself wouldn't protect them from attacks, but rather what they could do with it. The only alternative that explicitly shows something that could be used to protect them would be D.

Building walls, however, had little effect on attacks that were launched by sea, and they were many.

definitely a for sure. ill see and update if i get the answer